1. Field of the Invention
This invention relates to an improved process for the manufacture of aliphatic phosphonic acids, and more particularly to a process for the manufacture of a 2-haloethylphosphonic acid and still more particularly, to the manufacture of 2-chloroethylphosphonic acid.
2. Summary of Prior Art
It is known in the art, that certain aliphatic phosphonic acids, particularly 2-haloethylphosphonic acids and derivatives thereof, are valuable as plant growth regulators in the agricultural field. The chloro compound i.e., 2-chloroethylphosphonic acid has been used extensively as a plant growth regulator for increasing crop yield of, for example, pineapples, soybeans, and the like. The usefulness of these compounds is illustrated for example in the publication of Nature, vol 218, page 974 (1969) by Cooke and Randall; and U.S. Pat. No. 3,879,188 issued Apr. 22, 1985 entitled "Growth Regulation Process" invented by Fritz, Evans, and Cooke.
The manufacture of 2-chloroethylphosphonic acid (ETHEPHON) is well known in the art. British Pat. No. 1,373,513 describes a process for the manufacture of chloroethylphosphonic acid which comprises cleaving the diester with aqueous hydrochloric acid optionally in the presence of gaseous hydrogen chloride at a temperature of about 100.degree. C. and under an elevated pressure, continuously or discontinuously distilling off the 1,2-dichloroethane formed during the reaction and maintaining an elevated pressure by adding gaseous hydrogen chloride continuously or discontinuously during the reaction.
Another variation on the process of cleaving the diester is described in U.S. Pat. No. 3,808,265 which describes a process for the manufacture of 2-chloroethylphosphonic acid by reacting the diester and concentrated aqueous hydrochloric acid under pressure of injected hydrogen chloride gas sufficient to replenish the reacted HCl and maintain the concentration of the aqueous hydrochloric acid above about 23% at a temperature of about 100.degree. C. to 145.degree. C., cooling the reaction product to obtain a two-phase liquid system consisting of an aqueous phase containing 2-chloroethylphosphonic acid and an organic phase containing ethylene dichloride, drawing off the ethylene dichloride phase and recovering 2-chloroethylphosphonic acid from the aqueous phase.
All of the above processes involve the acid cleavage of the ester groups from the diester of the phosphonic acid to yield the corresponding phosphonic acid.
This cleavage reaction for the manufacture of phosphonic acid occurs in a stepwise manner. The diester is converted to the half ester which, in turn, is cleaved to the phosphonic acid.
The cleavage of the bis(2-chloroethyl)-2-chloroethylphosphonate (the diester), by known prior art methods utilizing anhydrous HCl, depending upon reaction conditions and concentrations, can produce from 45 to 84% 2-chloroethylphosphonic acid and from 3 to 30% of the mono-2-chloroethyl-2-chloroethylphosphonate (the half ester).
A major impurity in the manufacture of 2-haloethylphosphonic acid is the half ester of the corresponding acid.
It is highly desirable, when applying 2-haloethylphosphonic acid to plants, that the compositions applied be sufficiently free of impurities such as half esters of 2-haloethylphosphonic acids, to avoid the toxic or potentially toxic effects of these impurities on plants. It is thus highly desirable to either remove the half ester from the crude reaction mixture obtained from the cleavage of the diester of the corresponding acid or to obtain within reasonable operating parameters, i.e., time, pressures, concentrations, etc., substantially complete reaction of the diester and half ester to 2-chloroethylphosphonic acid.
Generally, attempts have been made to purify 2-haloethylphosphonic acid manufactured from the prior art processes.
U.S. Pat. No. 3,626,037 describes the extraction with benzene of the monoester of 2-chloroethylphosphonic acid from a reaction mixture containing the monoester and 2-chloroethylphosphonic acid. The disadvantages of using benzene as a commercially practical extracting solvent are that it is highly flammable, acutely toxic to humans, and gives poor phase separation when used as an extracting solvent for the half ester of haloethylphosphonic acid.
British Pat. No. 1,187,002 describes the extraction of the monoester of 2-haloethylphosphonic acid from the crude reaction mixture with a halo-hydrocarbon of from 1 to 8 carbon atoms, such as chlorobenzene, dichlorobenzene, chloroform, tetrachloroethylene, and preferably methylene chloride or ethylene dichloride. The disadvantages of using halo-hydrocarbons as commercially practical extracting solvents are that they are highly toxic to humans and a poor extracting solvent for the purification of 2-haloethylphosphonic acid.
One serious drawback with attempting to obtain the highly pure aliphatic phosphonic acids necessary for commercial use by the addition of an extracting process to the process of manufacturing is that this extracting process involves an additional step with its concomitant increase in cost, both in capital and operating expenses.
More recently, U.S. Pat. No. 4,064,163 describes in detail a process for the manufacture of an aliphatic phosphonic acid, particularly 2-chloroethylphosphonic acid, of the type wherein a diester of the aliphatic phosphonic acid, particularly bis-(2-chloroethyl)-2-chloroethylphosphonate, is reacted with anhydrous hydrogen halide, particularly hydrogen chloride, to produce a reaction product containing the aliphatic phosphonic acid, and the corresponding aliphatic halides, particularly ethylene dichloride.
In general the process provides:
(a) reacting the diester and the anhydrous hydrogen halide at a first temperature of at least 100.degree. C., at a low pressure and for a first period of time to form a first reaction product;
(b) subsequently reacting the first reaction product and the anhydrous hydrogen halide at a second temperature of at least 100.degree. C., at a high pressure, said high pressure higher than said low pressure, for a second period of time to form a second reaction product; and
(c) removing the aliphatic halides from at least one reaction product.
The process, disclosed in U.S. Pat. No. 4,064,163 is particularly related to a process for the manufacture of 2-chloroethylphosphonic acid utilizing the diester bis-(2-chloroethyl)-2-chloroethylphosphonate and wherein the anhydrous hydrogen halide is anhydrous hydrogen chloride. In general, the low pressure should be at least 14 psia and the high pressure at least 20 psia. Preferably, the low pressure is from about 14 psia to about 30 psia and the high pressure is from 30 psia to about 50 psia. In addition, the step of reacting the diester and the anhydrous hydrogen halide and the step of subsequently reacting the first reaction product and the anhydrous hydrogen halide are both accomplished at at least 100.degree. C. Preferably, the first and second temperatures are both from about 140.degree. C. to about 160.degree. C. The temperatures should not exceed 200.degree. C. Below 100.degree. C. the reaction kinetics are such as to cause the reaction to proceed too slowly, whereas above 200.degree. C. product and raw material decomposition interferes with the reactions.
Normally, the first period of time in the first step for the manufacture of 2-chloroethylphosphonic acid in the first step is usually at least about 25 hours and preferably from about 25 to about 35 hours. Further, the second period of time in the process for the manufacture of 2-chloroethylphosphonic acid is until the second reaction product contains a required concentration of at least about 80% by weight of 2-chloroethylphosphonic acid. The second period of time in the preferred process for the manufacture of 2-chloroethylphosphonic acid is at least about 10 hours. Preferably, however, the reaction time is from about 10 hours to about 16 hours. Commercially acceptable 2-chloroethylphosphonic acid for most agricultural uses is generally considered to have a minimum concentration of 85% by weight of 2-chloroethylphosphonic acid in the second reaction product. This can usually be accomplished within a second period of time of about 13 hours.
As will be discerned from the above, present techniques for producing aliphatic phosphonic acids and particularly 2-chloroethylphosphonic acids require a considerable amount of time in the order of about 35-50 hours or more to obtain product purity of at least 90%. This is in part mainly attributable to the batch mode of feeding starting materials to the reaction vessel. As a result of the batch mode of feeding of the starting materials, ethylene dichloride which is continuously evolved and discharged from the reaction vessel results in volume shrinkage of the reaction zone during the reaction period. The volume shrinkage is caused by a decrease in density and evolution of ethylene dichloride. At about 98% conversion, the final volume is only about 50% of the original. Thus it will be seen that the reaction vessel is not efficiently utilized for this type reaction.
It is thus an object of this invention to provide a novel process for manufacturing aliphatic phosphonic acids.
Another object of this invention is to provide a process for manufacturing highly pure aliphatic phosphonic acids at a low cost.
Another object of this invention is to provide a novel manufacturing process which is particularly suitable for manufacturing 2-haloethylphosphonic acids, and more particularly 2-chloroethylphosphonic acid.
Still another object of this invention is to provide a novel process which is particularly suitable for manufacturing, 2-haloethylphosphonic acids of highly pure nature under economically reasonable process parameters.